A. Field of the Invention
The present invention relates to Local Area Networks (LANs). More particularly, the invention relates to a LAN architecture specifically designed to be operated within a home environment, but may also be used in a small business setting. Home LANs may be used to connect computers, peripherals, TVs, and audio equipment, as well as less intelligent devices (appliances, thermostats, etc.), and provide connectivity to devices and networks outside the home (e.g., Internet and corporate LANs).
B. Description of the Related Art
Networks are collections of independent computers that communicate with one another over a shared physical connection, or network medium. Networks are often categorized as Local Area Networks (LAN) and Wide Area Networks (WAN).
1. Local Area Networks
Local area networks are usually confined to a specific geographic area, such as an office building. LANs, however, are not necessarily simple in design, and may link together hundreds of computers. The development of various standards for networking protocols has made possible the proliferation of LANs in organizations worldwide for business and educational applications.
Ethernet is a LAN networking protocol commonly utilized today. Ethernet typically utilizes a “star” or “spoke” topology, where each computer of the LAN is connected to other computers via a central hub. In such a configuration, each computer has its own private connection to the LAN and can be disconnected from the network without interfering with any other computer's connection.
Ethernet LAN technology was standardized by the Institute of Electrical and Electronics Engineers (IEEE) as the 802.3 specification entitled “Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications.” Initially, Ethernet technology used coaxial cable in a bus topology, however it has evolved to take into advantage of new technologies such as twisted pair cabling (10 Base-T), fiber optics (10 Base-FL), and 100 Mbps operation (100 Base-X, or Fast Ethernet). The current standard is known as IEEE 802.3u, the contents of which are hereby incorporated by reference. One limitation of 10/100 Base-T is the requirement for “home run” cabling, i.e., each device is connected back to a central hub, as opposed to “daisy chain” connections.
The Ethernet system consists of three basic elements: the physical medium; medium access control rules; and a packet format. The physical medium is used to convey Ethernet signals from one computer to another. The medium access control rules are embedded in each Ethernet interface, and allow multiple computers to access the shared Ethernet channel. The Ethernet packet, or frame, consists of a standardized set of fields used to carry data over the system.
2. Wide Area Networks
Interconnected LAN networks or individual users located in multiple physical locations are known as Wide Area Networks (WAN). The interconnections are performed via services such as dedicated leased phone lines, digital subscriber lines, dial-up phone lines, satellite links, and data packet carrier services. Wide area networking can be as simple as providing modems and a remote access server to allow remote users to dial in; or it can be as complex as linking hundreds of branch offices across the world using special routing protocols. Once type of WAN interconnection mechanism is Asymmetric Digital Subscriber Line.
3. Asymmetric Digital Subscriber Lines
Asymmetric Digital Subscriber Line (ADSL) is a communication system that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices, as opposed to multi-point, where numerous devices share the same physical medium.
ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by the ADSL modems—the POTS channel operates in the standard POTS frequency range and is processed by standard POTS devices after being split from the ADSL signal.
The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation.
4. Discrete Multi-Tone Modulation
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated using a type of Quadrature Amplitude Modulation (QAM). Alternative types of modulation include Multiple Phase Shift Keying (MPSK), including BPSK and QPSK, and Differential Phase Shift Keying (DPSK). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers. The symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the “bin” associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithm called Inverse Fast Fourier Transforms (IFFT).
The symbol period is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band.
Because the symbol time is relatively long and is followed by a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each subcarrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 subcarriers, each with a 4 kHz bandwidth. Each sub-carrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the sub-channels.
The sub-channels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a sub-channel, the data in each sub-channel can be demodulated with a small amount of interference from the other sub-channels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension appended at the end of each symbol to maintain orthogonality. Because of the periodic nature of the FFT, no discontinuity in the time-domain channel is generated between the symbol and the extension. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, sub-channel isolation is achieved.
5. Residential Phone Wiring
Plain Ordinary Telephone Service (POTS) operates over numerous types of existing wiring layouts. Typically, the topology is a star configuration, combined with daisy chained connections for some phones. The type of wiring is also random—twisted pair, untwisted, various gauges, various numbers of wires (with possible cross-talk)—which creates a wide variation in the channel characteristics. Furthermore, the topology changes from time to time as phones are connected, disconnected, etc.
Thus the channel characteristics are very noisy and distorted, including phase distortion (group delay) and severe reflections (echoes from signals bouncing off unterminated wiring segments) and spectral dips due to unterminated wiring stubs. In addition, signals associated with the analog phone service include 48 vdc, and 100 volt ring signals. Ring signals are not zero-crossing switched, so high-frequency noise is produced. Although in time, the POTS infrastructure will be rendered obsolete, any system operating over existing telephone wiring must contend with this environment. There exists a need for a communication system that operates reliably over standard telephone wiring found in residential environments, thereby eliminating the need for re-wiring a residence.